Hello guys. Sorry for the long vacation. Have been busy with RC planes. Our last topic was encoders and decoders in RC gear. Let's hightlight some points about the non- digital way of driving the servo. In the ordinary PWM system, the Tx sends about 60 frames of commands to many servos. The more the channels, the less command is given to a servo per unit time. Let's take a 4 channel system. The allocation is 2 MS for each channel totalling 8 ms and 10ms for the pause. That's 18ms divided into 1000MS in a second. At 20 MS, that would be 50 times a second. If you have 6 channels, that would have been 6 x 2m=12, plusa 10ms pause to make a total of 22 ms. There will be less than 50 c0mmands to a servo per second. Now, how does this affect the servo strength? Well, everytime a servo arm is pull out of its position, the pulses drives the motor to pull it back in. If we had more pulses, the pulling in would be finer. That is where the digital servos come in. In a digital servo, the number of pulses "correcting" the servo position is about 300 times vs 50-60 for the analog system. That is about five times. It's like a gear of 300 teeth vs one with 60. The drive is finer as it can correct a smaller error and in smaller units. The drawback is that digital servos also draw five times the current! How is this done. Well, it cannot be possibly converting the 60 pulses per second from the transmitter and convert it to 300 pulses. That is not possible. It became possible with PCM. Pulse code instead of pulse width. In PCM, the Tx sends a binary code to tell a CPU in the RX to position a servo. For example the Futaba 1024 PCM, the servo can take up 1,024 positions. And the beaty of it all, the TX only have to say it once, like sending in the code to take up the position 512 which is center. It's like the command is "Don't change position until I tell you". The local CPU will generate the pulse of 1.5MS but at 300 pulses per second. And that is why a PCM RX is immune to electrical noise since the code has to be exactly legible for it to recognize as a command. In PWM, a glitch can be read as a pulse. Theoretically, if you had switch off a PCM TX momentarily, the servos will remain in their position since no new command was received. This is not the fail safe feature in PCM. The failsafe feature is when a given setting for each of the servos at a certain moment was stored in the CPU in the RX. If the RX does not hear from the ground after X seconds, the CPU switches to this mode even when nothing was heard from the TX again. The plane took on a circling mode for example. Another thing that can be memorized by the CPU is say, a loop. Next time you press the repeat button, the plane does it again. It's like putting the music organ to recording. Pressing a button will play it again and again.

bk